One of technologies for bonding substrates is flip chip bonding. Flip chip is a system for bonding one substrate, i.e. a semiconductor chip, with another substrate (e.g. a printed circuit board or an interposer) through two-dimensionally arranged bump electrodes or bumps, without leading out lead lines from the one substrate. In other words, flip chip bonding is to bond bump electrodes on surfaces of the semiconductor chip and the other substrate, or to bond bump electrodes on a surface of one of the semiconductor chip and the other substrate to lead electrodes on the surface of the other.
Currently, it is usual to use flux in bonding bump electrodes to lead electrodes. Such flux is used to reduce and remove oxides on the surface of or within the bump electrodes and to prevent oxidation of the surfaces of the bump electrodes. However, flux remaining on the substrates after the bump electrodes are bonded to the lead electrodes will adversely affect the reliability of the semiconductor. In order to avoid such adverse effect, it is necessary to rinse out flux residues in a space between the semiconductor chip and the substrate so that there will be no flux remaining in the space.
However, it is expected that the space between a semiconductor chip and a substrate will be 50 μm or less in the future, and it will be very difficult to rinse out flux remaining in the space. It is also not easy to remove flux residue completely. Recently, many technologies for bonding bump electrodes to lead electrodes without resort to using flux are being proposed. For example, Patent Literature 1 discloses a technology in which an adhesive containing alcohol or organic acid as its principal ingredient is used to temporarily secure bump electrodes to lead electrodes, the bump electrodes are exposed to an atmosphere containing a free radical gas (hydrogen radicals) to reduce (i.e. remove) oxide films on the surfaces of the bump electrodes, and, then, a heat treatment is carried out to bond the bump electrodes to the lead electrodes.
Patent Literature 2 discloses a technology in which, after carrying out a treatment to reduce oxide films on the surfaces of electrodes, the electrodes are placed in position in the air, and they are melt in an oxygen-poor atmosphere.    Patent Literature 1: JP2005-230830A    Patent Literature 2: JP2007-266054A